This invention relates to a fuel cell system.
One type of a reformer for use in a fuel cell system uses a mixture of water and a liquid hydrocarbon such as methanol as an unreformed fuel. In this type of reformer, oxygen in air and the gaseous mixture of water and vaporized hydrocarbon are reacted in the presence of a reforming catalyst in order to produce a reformate gas (power-generating fuel gas) mainly comprising hydrogen. Both partial oxygenation reactions and water vapor reformate reactions are promoted in the reformer.
In a fuel cell system having the above type of reformer, it is necessary to vaporize the unreformed liquid fuel containing water and hydrocarbon. A vaporizer for vaporizing unreformed liquid fuel and a combustor supplying an amount of heat required for vaporization are provided for this purpose. The fuel in the combustor comprises air acting as an oxidizing agent and a hydrogen contained in anode effluent of the reformate gas which is not used in power generation in the fuel cell stack. The combustor is operated at a target operating temperature. Hence, it is necessary to regulate the flow rate of air and the flow rate of anode effluent in order to maintain the temperature of the combustor to a target operating temperature.
Furthermore the vaporizer is also operated at a target operating temperature in order to ensure vaporization of the unreformed fuel. The temperature of the vaporizer varies when the amount of unreformed fuel introduced into the vaporizer is varied. For example, when the flow rate of unreformed fuel introduced into the vaporizer undergoes a sharp increase as a result of a large increase in the required amount of power generation, the temperature of the vaporizer undergoes a sharp decrease and deviates from a target vaporizer temperature which corresponds to the required power generation amount. In some cases, the vaporizer temperature deviates from the permitted range of temperature bounded by upper and lower limiting temperatures.
Thus it is necessary to regulate the amount of combustion in the combustor so that an amount of heat canceling out this type of temperature variation is supplied from the combustor. Consequently the flow rate of anode effluent comprising hydrogen for the combustor is regulated on the basis of the amount of unreformed fuel introduced into the vaporizer. The control of this type of fuel cell system is disclosed in Tokkai Hei 2000-178001 published by the Japanese Patent Office in 2000.
This conventional technique introduces anode effluent which was not used in the fuel cell stack into the combustor through a return pipe which is connected to the inlet of the combustor and the discharge gas outlet of the fuel cell stack. Control of the temperature of the combustor is performed by regulating the amount of a liquid hydrocarbon, for example, when the amount of anode effluent is insufficient and the temperature of the combustor is not sufficiently high.
However in this conventional technique, the flow rate of anode effluent undergoes large fluctuations in an extremely short time period when it is attempted to immediately restore the temperature or when a temperature abnormality is detected. Furthermore variation in the internal pressure of the system is not taken into account when the flow rate regulation valve for anode effluent is opened and closed in rapid succession in order to control the temperature of the vaporizer or the combustor.
As a result, the pressure differential between the anode and the cathode in the fuel cell stack may exceed the permitted range of pressure. In this case, components other than the fuel cell stack, for example the reformer, may be damaged.
It is therefore an object of this invention to provide a fuel cell system which controls the temperature of a vaporizer or a combustor while maintaining the internal pressure of the fuel cell system to a permitted range of pressure.
In order to achieve above object, this invention provides a fuel cell system comprising: a vaporizer for vaporizing liquid unreformed fuel in order to produce an unreformed fuel gas; a reformer which reforms the unreformed fuel gas in order to produce a reformate fuel gas; a fuel cell stack which generates power using the reformate fuel gas; a combustor which produces energy by combusting combustible components in a discharge gas supplied to the combustor from the fuel cell stack, the combustor applying energy for producing the unreformed fuel gas to the vaporizer; and a programmable controller.
The programmable controller functions to control at least one of temperatures of the vaporizer and the combustor to their respective permitted temperature ranges, by varying the flow rate of the discharge gas to the combustor.
The controller sets a target variation over time for the flow rate, predicts the pressure of the reformate fuel gas based on the target variation over time for the flow rate, and changes the target variation over time for the flow rate when the predicted pressure for the reformate fuel gas deviates from a permitted pressure range.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.